Alkali metal chlorides, such as sodium chloride (NACl) and potassium chloride (KCl), are commercially electrolyzed using cation exchange membranes to make chlorine and either sodium hydroxide (NaOH) or potassium hydroxide (KOH). The state-of-the-art process for such a chloralkali electrolysis is membrane electrolysis, in which a non-porous membrane, typically a fluorocarbon membrane, separates the anode chamber and the cathode chamber. The use of a fluorinated ion exchange membrane as a means for separating the anode and cathode compartments of a fuel cell or an electrolytic cell, especially a chloralkali electrolytic cell, is well known. In an electrolytic cell, it is desired that the ion exchange membrane exhibit low cell voltage and high current efficiency, thereby enabling the electrolytic cell to be stably operated with low electric power consumption. In a fuel cell, it is desired that the ion exchange membrane exhibit high ionic conductivity, thereby enabling the fuel cell to be stably operated with high electric power output. Membranes are commonly reinforced with a chemical-resistant fabric to improve tear strength, burst strength and dimensional stability.
However, use of reinforcement within the membrane is not totally beneficial. One deleterious effect is that use of reinforcement such as fabric results in a thicker membrane, which in turn leads to operation at higher voltage because the greater membrane thickness has a higher electrical resistance.
In order to obtain a low cell voltage in a chloralkali cell along with good stability for handling the reinforcing fabric and the reinforced membrane, it is desirable to have an open reinforcing fabric and a thin membrane. A thin membrane requires a thin fabric and a small total thickness of the film layers used in laminating the reinforced ion exchange resin.
Efforts to lower the resistance by using thinner films in fabricating reinforced membranes are often unsuccessful because the film ruptures in some of the windows of the fabric during membrane fabrication, resulting in a membrane with leaks. ("Windows" means the open areas of a fabric between adjacent threads of fabric.) A membrane which leaks is undesirable as it permits anolyte and catholyte to flow into the opposite cell compartments, thereby lowering the current efficiency and contaminating the products made.
A second deleterious effect, which also leads to operation at higher voltage, is caused by a "shadowing" effect of the reinforcing members. The shortest path for an ion through a membrane is a straight perpendicular path from one surface to the other surface. Reinforcement members are uniformly fabricated of substance which is not ion-permeable. Those parts of a membrane where an ion cannot travel perpendicularly straight through a membrane, and from which the ion must take a circuitous path around a reinforcing member, are termed "shadowed areas". Introduction of shadowed areas into a membrane by use of reinforcement in effect leads to a reduction in the portion of the membrane which actively transports ions, and thus increases the operating voltage of the membrane. That part of the shadowed area of a membrane which is adjacent to the downstream side of the reinforcement members, "downstream" referring to the direction of the positive ion flux through the membrane, is termed the "blind area".
An open fabric is one in which the area of the fabric is at least about 150%, and preferably 200% or more, greater than the area of the permanent reinforcing yarns. In other words, it is a fabric with a large percentage of windows or open spaces and a small percentage of shadowed areas and blind spots. This is desirable because it is the open spaces which allow ions to readily pass during electrolysis. Thus, a more open fabric makes possible lower cell voltage and therefore a lower power consumption.
The simplest kind of fabric is one with a prior art plain weave, shown in FIG. 1. However, the yarns which comprise an open plain weave fabric tend to become disorderly and the fabric is not uniform. The resulting membrane may disadvantageously have non-uniform electrical properties across the membrane. In addition, the membrane may be susceptible to cracks, pinholes or wrinkling. In addition, if the fabric is made with high openness--a small number of yarns in each direction--the fabric lacks dimensional stability and may stretch out of shape. This is a serious problem during assembly of commercial electrolytic cells, particularly those which may require large membranes, some of which are as large as 1.5.times.3.7 meters (m), and those in which vertical assemble is employed.
In order to make a more open fabric with uniform open spaces than is feasible with a plain weave, considerable attention has been given to a leno weave fabric. A prior art leno weave is shown in FIG. 2. For example, U.S. Pat. No. 4,072,793 teaches the use of leno weave fabrics, including fabrics made from fibers of fluorocarbon polymer such as polytetrafluoroethylene ("PTFE"). However, as can be seen from FIG. 2, the fabric tends to be thick at the point where two warp yarns cross a filling yarn at about the same place, resulting in a triple cross-over point. It is also necessary to use fill yarns that are twice the denier of the warp yarns if the fabric is to have the same physical properties in both directions. Because fabric strength is not a limiting characteristic and 100 denier is presently the smallest PTFE commercially available, such leno weaves are stronger and thicker than necessary for their reinforcing function. Thick fabrics are generally considered undesirable because they require a large amount of polymer to cover the fabric on both sides of the membrane and cause large shadowed areas. If the yarn penetrates the surface of the membrane, it may cause leakage from one electrolyte to the other along voids that result because adhesion of the polymer to the yarn is imperfect. Leakage of the catholyte into the anolyte causes low current efficiency and high power consumption along with other problems. Leakage of anolyte into the catholyte may lead to amounts of chloride in the caustic product which exceed customer requirements.
Moreover, in leno weave the pairing of yarns in the warp tends to be imperfect and the windows in the membrane are not square. This disadvantageously increases the possibility of puckering of the membrane during operation, which may decrease the useful life of the membrane.
It is also possible to incorporate sacrificial fibers into the fabric. The sacrificial fibers provide mechanical strength and stability during handling of an ion exchange membrane, but may be removed during operation of the membrane so as to reduce interference with the transport properties of the membrane. The sacrificial yarns confer stability to an open (with respect to permanent, resistant yarns) plain weave fabric. The use of sacrificial fibers in cation exchange membranes is described in U.S. Pat. No. 4,437,951. Sacrificial yarns normally outnumber the permanent yarns 2-10:1 so they increase weaving time and material cost. In addition, sacrificial yarns undersirably leave channels in the membrane which cause leakage at the membrane edges, causing corrosion to the electrolytic cell and deterioration of the cell gasket. The channels may also be reservoirs of chlorinated brine which can cause cathode corrosion during shutdowns.
Therefore, there is needed a reinforced membrane which is flat, thin and has a large percentage of open spaces, provides good tear strength, burst strength and dimensional stability and retains the advantages of prior art membranes.
The present inventors have developed an ion exchange membrane which incorporates a leno weave fabric, especially if made of low denier yarns, which is a thin fabric, stable under various stresses even if the fabric is of high openness. The leno weave is one in which the warp yarns are arranged in pairs with one twisted around the other between picks of filling yarn as in marquisette. This type of weave prevents slippage and displacement of warp and filling yarns. Similarly, the reinforced membrane is stable during handling and installation, under the forces of shrinkage and expansion inside the electrolyzer, and during disassembly of the cell, allowing a higher percentage of the membrane to be reinstalled and reused.